Recently, the optical communication system has improved rapidly, so that communication speed has increased and a scale of the system has increased in size. An optical communication system has been used for a private communication such as LAN (local area network) and the like. Therefore, it is required that the system is fabricated to be low in cost and compact in size. In this situation, an optical module, for example an LD (laser diode) module and an APD (avalanche photodiode) module are important to meet the above requirements.
A first conventional optical module includes a module body, an optical device, a TEC (thermal expand core) fiber which is a part of a pigtail cord, and an aspherical lens for optically connecting the optical device and the TEC fiber. The TEC fiber is provided with a fiber end whose core diameter is expanded to face the aspherical lens. The fiber end is shaped to have an expanded diameter end by thermal treatment of about 1000.degree. C. in an electric furnace.
The conventional optical module is described in a report of the autumn national convention C-193 1990 by Electronic Information Communication Society, entitled "Coupling characteristic between an optical fiber having an expanded diameter end and an LD (LASER DIODE)" by Mr. Kuniharu Kato, et al.
In fabrication of the conventional optical module, the aspherical lens is set in a body, and the optical device is fixed to be aligned to the body. After that, the TEC fiber is aligned to the optical axis and is fixed to the body.
According to the conventional optical module, a coupling tolerance of the TEC fiber is increased, because the TEC fiber has the expanded diameter core end. However, there are disadvantages in that the TEC fiber must have a length of ten to tens of centimeters in order to function as a pigtail cord. Therefore, the TEC fiber is difficult to be mass-produced by using an electric furnace, because a long optical fiber for the TEC fiber can not be put in the electric furnace of normal size. As the result, fabrication cost of the optical module becomes high. Further, the optical module cannot be fabricated efficiently, because the TEC fiber (pigtail cord) is cumbersome when the TEC fiber is aligned to the optical axis. The above described optical module is defined to be "a pigtail type". On the other hand, if the optical module is a receptacle type in which a receptacle is formed on the body, a sleeve connected to a ferrule is necessary to be fixed to the body, so that the optical module cannot be fabricated with low cost.
Further, an optical system having both functions of communication and broadcasting has been proposed. In such an optical system, two optical fiber lines for up-and-down transmission are installed between a subscriber and a station. On the other hand, there are two more economic methods, a bilateral transmitting method by using a common wavelength and a multi-transmitting method by using two kinds of lights having different wavelengths.
A conventional optical module unit includes an LD (laser diode) module of pigtail cord type or receptacle type, an APD (avalanche photodiode) module, an optical coupler for coupling the LD module and the APD module to a common optical fiber, and two optical connectors for connecting the LD module and the APD module to optical fibers connected to the optical coupler.
According to the conventional optical module unit, a bilateral communication can be carried out.
However, there is a disadvantage in that the size of the optical module unit is large and the cost is high, because the optical connectors are necessary to connect each module to the optical coupler.